Illumination method and device

ABSTRACT

A method for furnishing a perceptor with apparently continuous illumination over an extended target area, in which at any instant only part of said area is illuminated, but every part thereof is intermittently and repeatedly illuminated by discontinuous flashes. With regard to any one part of said target area, the flashes are repeated at time intervals not less than the decay period of the response elicited in the perceptor. 
     The method is performed by means of a device which comprises a beam-generating arrangement ( 1 ) that focuses a beam of radiation ( 5 ) upon a rotatably-mounted light-deflector ( 6 ), which is rotated under control at a suitably high speed by mechanical means ( 9, 10, 11  and  12 ) so as repeatedly to illuminate a target area ( 15 ) with a narrow flash to radiation ( 16 ).

This invention concerns an illumination method and device.

Broadly-speaking the invention relates to a method whereby brightillumination provided by a concentrated, narrow beam of light or otherelectromagnetic radiation can apparently be disseminated, withcomparable intensity, over a much wider area. The invention moreoveralso concerns a light disseminator device which is a combination oflight deflector(s) with other means and which is able, in co-operationwith a light source, to provide relatively high-intensity apparentillumination over a widespread target area, that is to say wide-arcillumination apparently more intense than could be spread over the sametarget area by the light source unaided by the device.

It is a commonplace that light emanating from a light source willnormally be radiated therefrom broadcast in all directions, withcorrespondingly low intensity in any one direction. It is however alsoone of the most basic achievements of optics that light emanating fromsuch a light source can be concentrated and directed by means of asuitable reflector (thus a mirror or system of mirrors) and/or refractor(thus a lens or system of lenses) into a narrow beam, which castsillumination of relatively much greater intensity in a chosen directionthan would otherwise have been broadcast in that direction—but of courseat the expense of diminishing or denying illumination in otherdirections. It seems that one is faced with an apparently inescapablechoice—between relatively low-intensity illumination over a wide area onthe one hand, or relatively high-intensity illumination over a narrowarea on the other. And this is indeed the inescapable choice, when theintensity of illumination is perceived entirely objectively—there is noavoiding the laws of science, and one does not get something fornothing.

It is known, however, that the perceived intensity of illumination is incertain circumstances not objective but can be quite subjective. Thisphenomenon is called persistence of vision, and refers to how the humaneye can be fooled into perceiving continuous illumination even if it isin fact discontinuous, i.e. rapidly repeated flashes of illumination.Therefore it is possible to produce in the eye of an human (or animal)perceptor an illusion of wide-arc, relatively high-intensity apparentillumination if a narrow, concentrated beam of such relativelyhigh-intensity illumination is intermittently but repeatedly swept atsufficiently high frequency across a wide target area.

Various methods of overcoming the objective problem, which utilise thisphenomenon, have been suggested, and the most pertinent of these havebeen outlined below.

U.S. Pat. Nos. 3,865,790 and 4,153,926 disclose methods and deviceswhich have tried, with only partial success, to solve the problem bytaking a device that produces a beam of light, and then rotating theentire assembly at high speed. Similarly British Patents No. 694,357 andNo. 1,083,492 both also relate to devices where the light source and thebeam concentrating means are rotated together.

Whilst fine in concept, this type of device is rather lacking inpractical feasibility. For a start the beam produced tends to be a discin overall configuration and this is not by any means ideal. The sourcewill only cast light on a given point once (per beam that is produced)per revolution of the source. More importantly however, in devices ofthis general type, the light source can be one of relatively high powerand therefore produce several beams, or it may be confined to producingone beam only and therefor require a less powerful source of light.Naturally when more beams than one are produced, and are able to scanacross the target area, then the speed of rotation of the source can bereduced, but even so it will still be required to rotate at high speed.One is faced with the dilemma that if the amount of beams produced isincreased, then the speed of rotation can be decreased but the size ofthe device that must be rotated is increased—whereas conversely theopposite of course is true in that the size of device can be kept downby using fewer beams, but then the speed at which the device must spinis dramatically increased.

These considerations mean that any design of this type must be fairlycumbersome to contain all the features required to rotate a large andcomplex object at high speed. For instance it requires fairly complex,and hence unreliable, wiring mechanisms to electrically link therotating bulb to the power supply. Additionally the whole rotating partmust be carefully balanced to prevent vibration and the problemsassociated with it.

The most important point is however that, during high speed rotation,the filament of the bulb can be forced out of alignment with the optics,due to the centrifugal forces. This is hard to avoid because a filamentmust by design be of narrow diameter and hence flexible.

In an attempt to overcome some of the problems associated with the abovedisclosed methods, devices wherein the light source was held stationaryand the beam producing means were rotatable therearound were insteadproposed. In British Patent No. 488,616 a device with lens arraysrotating about a light source was disclosed. Additionally in BritishPatent No. 520,079 a fixed light source with a set of rotating parabolicmirrors located around it was proposed. Both these devices suffer fromthe problem of having to rotate the beam means around the light sourceat high speed, but in close proximity to the bulb. This is especially aproblem of the device of GB 520,079 which had at least two back-lessparabolic reflectors joined around the light such that they projected atleast two beams of light from the source. This has the effect ofproducing a weak source of light so that the overall lighting phenomenonis diminished.

Various other methods have been employed in an attempt to achieve theproposed objectives, and they have for example, involved a largerotating tower with complex internal reflectors as in GB 558,828; orthey have used vibrating mirrors, light source and rotating prisms toscan light over a small area as in GB 951,604.

All the above have failed to effectively overcome the problemsassociated with attempting to achieve the objectives of the presentinvention, or indeed for that matter the objectives they set themselves.Indeed the very fact that none of them ever caught on, gives testamentto their lack of effectiveness. The present invention, on the otherhand, provides a convenient and effective means of achieving thoseobjectives and overcoming the problems.

Therefore, according to this invention in its broadest aspect, there isprovided a method of furnishing a perceptor with apparently-continuousillumination by electromagnetic radiation to which the perceptor isresponsive over an extended target area, in which a rotatable reflectoris used to deflect a relatively narrow beam of radiation from one pointto another over a relatively wide target area, whereby at any instantonly part of said area is illuminated with said radiation but every partthereof is intermittently and repeatedly illuminated by discontinuousflashes of said radiation, said flashes being as regards any one part ofsaid target area repeated at time intervals not less than thedecay-period of the response of the perceptor to that radiation.

The terms “radiation” and “reflector” used above, and hereinafteremployed for convenience, refer respectively to any suitableelectromagnetic radiation that may be efficiently reflected, and to areflector capable of reflecting said radiation.

It is currently envisaged that the electromagnetic radiation employedwill be in the ultraviolet, visible and/or infrared ranges, thuscorresponding to wavelengths of say from 1 nm up to about 5 nm. For thepurposes at present contemplated it will be preferable to use visiblelight with wavelengths in the range of from about 380 nm up to about 780nm, and/or actinic radiation i.e. light in the violet and ultra-violetregions of the spectrum which will bring about chemical or photochemicalchanges, and may be regarded as corresponding to wavelengths of from 4to 600 nm. Of course the term “ultra-violet (or UV) radiation” refers tothe non-visible part of actinic radiation, and may be regarded ascorresponding to wavelengths of from 4 to 400 nm., and more especially325-365 nm. Thus overall the preferred visible and actinic radiation foruse in the method of the invention corresponds to wavelengths in therange of from 4 nm up to 780 nm. The electromagnetic radiation employedmay be coherent, subject to the normal considerations governing itsgeneration and use; but as currently envisaged will usually be normal,incoherent radiation.

Where the context so allows, the term “perceptor” as used hereinincludes not only the human (or other animal) eye responsive in thevisible light range but also non-animal (e.g. electric and/orelectronic) perceptor instruments responsive in the visible and/or thenon-visible radiation ranges. It moreover also includes part-human (orother animal) and part-instrumental perceptors, as for instance whennon-visible radiation is perceived initially by an instrument responsivethereto but then converted within that instrument into a secondary imagein the visible light range and thus perceptible by the human (or otheranimal) eye of an ultimate observer.

The decay of the response of any perceptor will generally beexponential, and of course the term “decay-period” is not here used inan extreme theoretical sense which could include almost infinite periodsas the response approaches zero but in its practical sense whichembraces only perceptor-responses that are useful for their intendedpurpose. On an admittedly arbitrary basis the outside limit of therelevant decay-period can be defined as that over which the response ofthe perceptor falls to 30% of the maximum response of the perceptor tostimulation by that radiation. For all currently-envisaged purposes thedecay-period should be set at that during which the perceptor-responsefalls to no less than 50% of maximum, and it is believed that the bestresults will be achieved when the relevant decay-period is set to end ata level of 80% or even 90% of maximum response.

In order to reduce or avoid any sensation in the perceptor of flickeringin the perceived illumination it is quite desirable that the flashes ofillumination should be repeated as regards any one part of the targetarea at least twice during the decay period, and (within experience sofar) it is best if they are repeated substantially three times duringthat period. When the illumination is in the visible range and theintended perceptor is the human eye these preferences correspond roughlywith the flashes of visible light being desirably repeated at leasttwice every one-tenth of one second, and best repeated substantiallythree times every one-tenth of one second.

According to another preferred aspect of this invention there is alsoprovided a light disseminator, for use in carrying out the method hereindisclosed, which comprises means operable to direct a beam of light sothat it impinges upon a rotatably-mounted light-deflector, saidlight-deflector being arranged and disposed so that dependent upon itsrotational position it will deflect the light-beam to one point oranother around an arcuate target area centred upon the rotatabledeflector, and means operable to rotate the light-deflector so that itsweeps the deflected beam around said arcuate target area, at arotational rate such that any given part of the arcuate target area isintermittently but repeatedly illuminated by discontinuous flashes oflight provided by the deflected light-beam at time-intervals of not morethan one-tenth of one second.

In this case, the perceptor is to be the human eye, and thetime-intervals should preferably be not more than one-thirtieth of onesecond, and possibly or even desirably still less.

Of course, the beam-directing means will desirably be so disposed andarranged as normally to direct a beam of substantially parallel light toimpinge upon the rotatably-mounted mirror, but it is for some end-usesadvantageous also to provide means for adjusting the arrangement out ofits normal disposition so as either to converge or to diverge theotherwise substantially parallel light-beam.

The beam-directing means preferably will comprise means for mounting alight-source, and a concave reflector mounted adjacent to saidlight-source on its side remote from the light-deflector so as to assistin directing the desired parallel light-beam to impinge upon thelight-deflector(s).

Alternatively or in addition the beam-directing means may comprise meansfor mounting a light-source, and a convex lens or lens system mountedbetween said light-source and the light-deflector so as to assist indirecting the desired parallel light-beam to impinge upon thelight-deflector(s).

The light-disseminator will normally include an electrically-operableincandescent light-source supported in the mounting means, and thereprovided with electrical connections adapted under control to operatethe incandescent light-source. The light-source advantageously is orincludes a single-filament incandescent light bulb so supported in themounting as to dispose the filament with its axis normally vertical.

The light-deflector may be a refractor, e.g. a multi-sided-prism, butexperience so far suggests that it is advantageously a rotatably-mountedreflector, usually indeed a multi-faceted reflector. For the purposescurrently envisaged the rotational axis of the light-deflector(s) shouldin normal use be disposed vertically.

In the simplest arrangement the multi-faceted reflector willadvantageously be a double-side plane mirror. With such an arrangement,and in an ideal set-up wherein a beam of truly parallel light from atruly linear source is incident upon a plane mirror of the same depth asthe beam, then the reflected beam will be neither divergent norconvergent, and thus will have the same depth as the incident beam.Therefore on rotation of the mirror the reflected beam will be sweptaround a substantially 360° arc, creating at any given instant acorresponding small patch of high-intensity illumination, (having thesame depth as both the incident beam and the linear source) at oneparticular point on the 360° arc centred on the rotating mirror. Inpractice it is however effectively impossible to achieve such an idealset-up, and there is an inevitable tendency for the beam incident on themirror to include some stray, non-parallel light—and in that event thebeam even when reflected from a plane mirror will to some extent beslightly divergent. Nevertheless when using a beam of parallel light anda plane mirror most of the light is concentrated in thepreviously-mentioned small patch, and due to persistence of vision in anhuman observer's retina it will be perceived as a fairly thin, flat“band” of illumination around the rotating mirror, so-to-speak in a sortof horizontal disc.

Dependent upon requirements, it is possible either to accentuate thetendency for the beam to diverge or to try to counteract it.

Thus, in order to promote a wider band of illumination thelight-deflector can be so constructed and arranged that it encouragesthe substantially-parallel light-beam impinging thereon to becomedivergent in the vertical planes containing the rotational axis of thelight-deflector, e.g. by making the light-deflector a slightly-convexmirror.

Conversely, if it should be wished to concentrate the illumination intoa still narrower band, then the light-deflector can be so constructedand arranged that it counters any tendency for thesubstantially-parallel light-beams impinging thereon to becomedivergent, or indeed even forces it to become convergent, e.g. by makingthe light-reflector a slightly-concave mirror.

The transverse dimensions of the light-deflector in the plane normal tothe impinging light-beam will desirably exceed the width of thatlight-beam, so as to ensure that the full width of the light-beam isdeflected thereby for so much as possible of its rotation. On the otherhand the light-deflector would have to be of infinite width if it wereto be capable of deflecting the full width of the incident light beamthroughout its entire rotation, which of course is absurdly impossible.

Balancing these considerations, it currently appears that for practicalpurposes the width of the light-deflector (normal to the incident beam,and in the plane normal to its rotational axis) should conveniently bein the range of from about 1.12 to about 2.24 times the width of thatbeam. On a somewhat arbitrary basis, it is currently thought best if thewidth of the light-deflector is substantially 1.4 times the width of thebeam.

The light disseminator of this invention may be embodied in various waysaccording to the end-use envisaged. Possible uses seem very extensive,and have not yet been fully explored, but fall broadly into twocategories. In one category of end-use the ultimate observer carries thedevice himself or for instance upon a vehicle, and thus requireswide-arc but still partly-directional illumination ahead of him, e.g. inthe manner of a hand-held torch or a vehicle-mounted headlamp. Inanother category of end-use the ultimate observer wishes to set up thedevice to provide high intensity all-round illumination, eithertemporarily as for instance at the scene of an accident or otheremergency or on a more permanent basis as for instance in sportingarenas or other public concourse areas.

In order that the invention may be well understood various simpleembodiments thereof will now be described in more detail, though only byway of illustration, with reference to the accompanying schematicdrawings (in which so far as possible the same reference numerals havebeen used for the same parts in all the various figures) as follows:

FIG. 1 is a perspective view of the basic elements of alight-disseminator arrangement in accordance with this invention, laidout diagrammatic ally in a manner intended to facilitate understandingof its principle of operation rather than as it would be actuallyembodied in a commercial construction;

FIG. 2 is a plan view of a slightly more elaborate but basically similararrangement to that shown in FIG. 1 mentioned above;

FIG. 3 is a diagram also in plan view which indicates how rotation ofthe light-deflector sweeps the deflected light beam and thus the patchof instantaneous illumination around an arc of substantially 360°centred upon the rotational axis of the light-deflector;

FIG. 4 is a diagrammatic and exaggerated representation of analternative and sometimes desirable double-sided light deflector for usein the arrangement of FIGS. 1 to 3, which in place of plane mirrors usessemi-convex mirrors, i.e. mirrors which are convex in the vertical planethrough their rotational axis but planar radially thereof;

FIG. 5 is a still-diagrammatic, partly cut-away, perspective andpart-exploded view of a more practical embodiment of the basic lightdisseminator illustrated in FIGS. 1 to 4, intended to directillumination over a wide but not full 360° arc, rather in the manner ofa hand-held torch or car headlight;

FIG. 6 is a simplified, plan view of the embodiment of FIG. 5, with therespective light-source and spinning light-deflector compartmentsjuxtaposed (rather than exploded) and with their transverse dimensionsmore realistically adjusted relative to each other;

FIG. 7 is a similar plan view of the embodiment of FIGS. 5 and 6, whenmounted within a transparent housing, as they might be in an hand-heldtorch or, more especially, in a single car-headlight which affordswide-angle, bright, but still partly-directional illumination ahead andto each side of the observer carrying the torch or seated in thevehicle; and

FIG. 8 is a side-elevation, partly in cross-section, of an alternativeembodiment of combined light source and rotatable light deflector,intended to provide illumination around a full 360° arc.

Referring first to the schematic lay-out illustrated in FIGS. 1 to 3, anelectric light-source generally indicated 1 has a vertically-disposed,substantially linear incandescent filament 2, and is interchangeablysupported in suitable fittings (not shown) and supplied with power viaelectric leads 3. The light-source 1 is positioned with the verticalaxis of filament 2 at the focus of a semi-parabolic reflector 4, that isto say one which is parabolic in the horizontal plane but planar in allvertical planes, and directs a narrow but deep beam of substantiallyparallel light, approximately rectangular in cross-section, in thedirection of arrow 5 onto a double-sided reflector generally-indicated6, mounted on a rotatable, vertical spindle 7.

In the slightly more elaborate embodiment illustrated in FIG. 2, thearrangement also includes a centrally-planar but peripherally convexlens 17 positioned between the light source 1 and the rotatable lightdeflector 6, the convex periphery of which tends to collect stray,non-parallel light emergent from the parabolic mirror 4 and converge itinto parallel beam 5.

The top and bottom ends of spindle 7 are rotatably supported in journals8 a and 8 b, and the spindle 7 is provided with a driven pulley-wheel 9interconnected by belt 10 with the drive pulley-wheel 11 of an electricmotor 12 supplied with power via leads 13.

When power is connected to light-source leads 3 and motor leads 13 thelight generated by the filament 2 is concentrated into a narrow beamwhich is directed onto the rotating double-sided mirror 6 and theredeflected, e.g. in the direction of arrow 14, but as the spindle-mountedmirrors 6 are rotated the deflected beam is swept around in asubstantially 360° arc, partially indicated 15.

At any given instant the beam of light 14 will illuminate only a smallpatch e.g. as indicated at 16, that patch being illuminated at thatinstant with the full intensity of which the particular arrangement iscapable—but the illuminated patch will sweep around arc 15 at arotational speed directly related to that imparted to the spindle 7 bythe driven pulley-wheel 9, drive belt 10, drive pulley-wheel 11 andmotor 12. When the reflector employed is double-sided (as in all ofFIGS. 1 to 7) the sweep-rate will be twice the rotational speed of thespindle. The retina of the eye of the observer will perceive the patch16 at its full illumination no matter where it finds itself, and due topersistence of vision will continue to respond to that level ofillumination for about {fraction (1/10)}th of a second. Providedtherefore that the patch 16 is re-illuminated by the rotating beam atleast every {fraction (1/10)}th of a second the retina of the eye willperceive patch 16 as if it were steadily illuminated at the full levelof which the arrangement is capable, and this no matter where the patch16 under discussion is located around the 360° arc centred on therotating spindle 7.

Thus by driving motor 12 at such a speed as to sweep the beam around thearc at least once every one-tenth of one second the illustratedarrangement can persuade the eye of an observer to perceive thefull-level illumination of a narrow beam as if it extended all the timearound the full 360° arc. With the double-sided mirror arrangement ofFIGS. 1 to 3 this requires the motor 12 to rotate the spindle 7 at arate of at least 300 revolutions per minute (rpm) in order to achieve asweep-rate of at least 600 rpm.

FIG. 4 illustrates (in an exaggerated manner) a modification of thetwin-mirror arrangement shown in FIGS. 1 to 3, in which thespindle-mounted, double-sided plane mirrors 6 there shown are herereplaced by semi-convex mirrors, so that the impinging beam 5 isdiverged thereby into a broader band of illumination.

Referring now to FIG. 5, this shows (still rather schematically) a morepractical embodiment in which as before a vertically-disposed linearlight source 2 is supported between sockets 18 a and 18 b in alignmentwith the focus of the semi-parabolic reflector 4. By means of thesockets 18 a and 18 b the light source is thereby connected to electricleads 3. Unlike the previously-described arrangement this light-sourceassembly is provided with a transparent front cover-plate 19, formed ofglass or “Perspex” (Registered Trade Mark) or some similar rigidtransparent material.

The double-sided planar light-deflector 6 is, as in the previousembodiment, mounted on vertical spindle 7 rotatably supported betweenupper journal 8 a and a lower journal (not shown). The lower end ofspindle 7 is provided with a circular metal disc 19, whose function willbe explained below. The light-deflector assembly comprising double-sidedmirror 6, spindle 7, journals 8 a and 8 b (not shown) and the metal disc19 is however, unlike previously-described arrangements, housed within atransparent, evacuated housing 20, again formed of glass or “Perspex”(Registered Trade Mark) or some other rigid transparent plasticsmaterial.

Evacuation of the housing 20, even if less than total, reducesair-resistance to the rotation of the spindle and the double-sidedmirror mounted thereon—but of course introduces difficulties in drivingrotation of this light-deflector assembly 6. In this embodiment howeverthe metal disc 19 within the housing 20 serves as the rotor member of anelectrical induction motor, the stator 21 of which is mounted beneaththe rotor 19 but outside the housing 20. The stator member 21 is poweredvia electrical leads 3. Obviously using such an induction motor solvesthe problem of rotating the light-deflector assembly within housing 20,but necessitates supplying alternating current (ac) via leads 3 to thestator member 21. For certain purposes (e.g. in smalllight-disseminators akin to a hand-held torch or lantern) the need forthe use of ac is a complication which may be undesirable—but it can besolved even when the electric power supply is derived from a dc sourcesuch as a battery by interposing an inverter (not shown) between thepower source and the stator member 21.

The kind of arrangement described and illustrated with reference to FIG.5 above is basically advantageous because it enables the light-deflectorassembly to be housed within an evacuated enclosure thus reducingair-resistance to the rotation of the double-sided mirror 6 and therebyreducing power consumption and/or increasing the speed of rotation ofthe deflected light beam. This construction moreover facilitatesexchange of either the light-source assembly or the light-deflectorassembly, when either of them becomes defunct and in need ofreplacement.

FIG. 6 shows the assemblage of the light-source and the light-deflectorinto an unit, the relevant dimensions being approximately correct. Itwill be seen that the width of the twin-mirrors is about 1:4 times thewidth of the beam emergent from the light-source aperture, so that whenthe mirrors 6 are at an angle of about 45° the full beam-width is stillaccommodated within the available width of the mirrors.

FIG. 7 shows the assemblage of FIG. 6 mounted with a transparent housing22, such as might serve as a single, possibly roof-mounted headlight fora motor vehicle providing excellent illumination not only ahead of thevehicle (not shown) but also to both sides of it over a wide arc, as forinstance shown by arrows 23.

A quite different embodiment of light-disseminator, specificallyintended to provide all-round illumination, is shown in FIG. 8. Here thelight source 2 is an annular fluorescent tube mounted at the focus of anannular parabolic mirror 4, the annular light source 2 and parabolicmirror 4 being arranged around the vertical spindle 7 supported in ajournal 8 driven by bevel-gear 24 which in turn is driven by meshingbevel-gear 25 driven by electric motor 12 powered via leads 13.

The light from source 2 is directed upwardly by parabolic mirror 4 toimpinge upon a multi-faceted mirror 26, each facet being disposed at asuitable angle (e.g. 45°) to the vertical. Thus the parallel lightdirected upwardly from light source 2 and parabolic mirror 4 isreflected by the off-vertical mirror facets 26 into approximatelyhorizontal beams as indicated by arrows 27. Although for simplicity ofillustration this is not shown in FIG. 8, it should be noted that eachof the mirror facets 26 can advantageously be fluted.

It should at this point be observed that the embodiment of FIG. 8results in creation of not just one patch of reflected light but as manydifferent patches of reflected light as correspond to the number offacets 26 on the rotating mirror assembly.

It will be appreciated that to achieve the illusion of wide-arc highintensity illumination what is necessary is that any given part of thetarget to be illuminated shall be thereby repeatedly illuminated atintervals not greater than {fraction (1/10)}th second, and when a singlereflector is rotated to sweep a single beam around a 360° arc the rateof revolution of that single reflector must therefore be at least 600revolutions per minutes (rpm)—but the requirement relates to thefrequency with which any given patch of the target area is illuminated,and is not necessarily directly dependent on the rate of revolution ofthe spindle. In the case of the embodiment of FIG. 8, if themulti-faceted mirror has x facets then the minimum rate of revolution ofthe mirror assembly needed to achieve the illusion is 600/x revolutionsper minute.

What is claimed is:
 1. A method of furnishing a perceptor withapparently-continuous illumination by electromagnetic radiation to whichthe perceptor is responsive over an extended target area, in which arotatable double-sided reflector is used to deflect a relatively narrowbeam of substantially parallel radiation incident thereon substantiallynormal to its axis of rotation from one point to another over arelatively wide target area, whereby at any instant only part of saidarea is illuminated with said radiation but every part thereof isintermittently and repeatedly illuminated by discontinuous flashes ofsaid radiation, said flashes being as regards any one part of saidtarget area repeated at time intervals not less than the decay-period ofthe response of the perceptor to that radiation.
 2. A method as claimedin claim 1, in which the electromagnetic radiation employed has awavelength in the range of from 1 nm up to 5 nm.
 3. A method as claimedin claim 1, in which the flashes are repeated as regards any one part ofthe target area at least twice during the decay period.
 4. A method asclaimed in claim 1, in which normal, incoherent electromagneticradiation is used, the radiation employed lies in the visible range witha wavelength of from 380 nm to 780 nm, the perceptor is or includes theeye of an human observer, and the flashes of visible radiation arerepeated at least twice every one-tenth of a second.
 5. A lightdisseminator which comprises means operable to direct a beam ofsubstantially parallel light so that said light impinges upon arotatably-mounted double-sided light-deflector substantially normal tosaid light-deflector's axis of rotation, said light-deflector beingarranged and disposed so that dependent upon the rotational position ofsaid light-deflector, said light-deflector will deflect the light-beamto one point or another around an arcuate target area centred upon therotatable deflector, and means operable to rotate the light-deflector sothat said light-deflector sweeps the deflected beam around said arcuatetarget area at such a rotational rate that any given part of the arcuatetarget area is intermittently but repeatedly illuminated bydiscontinuous flashes of light provided by the deflected light-beam attime-intervals of not more than one tenth of a second.
 6. Alight-disseminator as claimed in claim 5, which also comprises means foradjusting the arrangement out of its normal disposition so as either toconverge or to diverge the otherwise substantially parallel light-beam.7. A light disseminator as claimed in claim 5, in which thebeam-directing means comprise a mounting for a light-source, and aconcave reflector mounted adjacent to said light-source on its sideremote from the light-deflector so as to assist in directing the desirednormally parallel light-beam to impinge upon the light-deflector.
 8. Alight disseminator as claimed in claim 5, in which the beam-directingmeans comprise means for mounting a light-source and a convex lens orlens system mounted between said light-source and the light-deflector soas to assist in directing the desired normally parallel light-beam toimpinge upon the light-deflector.
 9. A light disseminator as claimed inclaim 7, which includes an electrically-operable incandescentlight-source supported in the mounting, said light source including asingle-filament incandescent light bulb so supported in the mounting asnormally to dispose the filament thereof with its axis vertical, saidlight-source being there provided with electrical connections adaptedunder control to operate said light source.
 10. A light disseminator asclaimed in claim 5, in which said light-deflector has a transversedimension measured in a plane normal to said light-deflector's axis ofrotation and said light-beam has a width, also measured in said plane,said transverse dimension of said light-deflector exceeding said widthof said light beam in said plane by a factor in the range of from 1.12to 2.24.